Some doubters are real conspiracy nuts and no amount of argument or evidence will convince them. They are a lost cause. However, there is one piece of evidence still in existence that can convince the more reasonable doubter, namely the retroreflectors. They are used even today by the McDonald Observatory in Texas, for example.

Of course, the first thing doubters do is question whether a retroreflector is necessary to receive a return signal, thus casting its existence into doubt. Fortunately, the numbers show how such a reflector is mandatory for the system to work as it does.

This page is an attempt to show the calculations and numbers resulting from the specifications and measurements involved in this laser range finding activity.

If you see an error or inconsistency, please email me (cfb 002 at austin dot rr dot com). Form the address correctly for email (remove the spaces and substitute '.' for dot, etc.). Apologies for this. It is so munged to foil the damned email spammers.

- Albedo of Moon Surface = 0.073
- Distance to Moon = 3.6x10^8 Meters
- Laser Pulse Energy = 1500x10^-3 Joules
- Laser Pulse Duration = 200x10^-12 Seconds
- Laser Pulse Rate = 10 Hz
- Laser Pulse Power = 7.5x10^9 Watts
- Average Laser Power Output = 15 Watts
- Diameter of Laser Spot on Moon = 7 Kilometers
- Diameter of Retroreflector Spot on Earth = 20 Kilometers

- Collecting Area of Telescope = 0.37 Square Meters (ie, a telescope with ~0.75 meter diameter mirror obstructed by a 15cm secondary)
- Telescope Optical Path Efficiency = 90%

- Collecting Area of Retroreflector = 0.42 Square Meters
- Retroreflector Efficiency = 90%

- Given the efficiency of the scope, the power of the beam
output by it is:

15 Watts * 0.9 = 13.5 Watts - Given the Albedo of the Moon, the average power of the
reflected light is:

13.5 * 0.073 = 0.9 Watts - The area of the shell of illumination reaching the Earth's
distance is:

2 * Pi * (3.6x10^8)^2 ~= 8.1x10^17 Square Meters (a half sphere) - The collecting area of the receiving telescope is 0.34 Square
Meters. Thus, the ratio of collected light to total light is:

0.34 / 8.1x10^17 ~= 4.2x10^-19 [from 3)] - The average power of the energy entering the scope is:

0.9 * 4.2x10^-19 ~= 3.8x10^-19 Watts [from 2. and 4.] - Thus, given the scope optics efficiency, the total power
collected is:

0.9 * 3.8x10^-19 ~= 3.4x10-19 Watts [from 5]#### With Retroreflector

Now we'll look at the case where the laser illuminates a Lunar retroreflector. For simplicity's sake, we'll ignore the energy returned from the Lunar surface proper and focus just on the energy returned by the retroreflector. - The area of the illuminated spot on the Moon in square meters
is:

Pi * 3500^2 ~= 3.8x10^7 Square Meters - The ratio of the retroreflector area to total illuminated spot
area is:

0.42 / 3.8x10^7 ~= 1.1x10^-8 [from 7.] - The power of the laser light collected by the retroreflector
is:

13.5 Watts * 1.1x10^-8 = 1.5x10^-7 Watts [from 1 and 8.] - The power returned by the 90% efficient reflector is:

1.5x10^-7 * 0.9 ~= 1.3x10^-7 Watts [from 9.] - The area of the retroreflected spot on the Earth is:

Pi * 10000^2 ~= 3.1x10^8 Square Meters - Again, the collecting area of the receiving telescope is 0.34
Square Meters. Thus, the ratio of collected retroreflected light
to total light is:

0.34 / 3.1x10^8 ~= 1.1x10^-9 [from 11.] - The average power of the energy entering the scope is:

1.3x10^-7 * 1.1x10^-9 ~= 1.4x10^-16 Watts [from 10. and 12.] - Thus, given the scope's efficiency, the average power
collected when using a retroreflector is:

0.9 * 1.4x10^-16 Watts = 1.3x10-16 Watts [from 13] - Using 10 * log (Pwr1 / Pwr2), the power dB difference is:

10 * log ( 1.3x10^-16 / 3.4x10^-19 ) ~= 26dB [from 6. and 14.]

In all the above calculations, remember that it is the relative difference between the retroreflector and non-retroreflector cases being emphasized here. The absolute measurements of the return signals are not that important as I have not included the many environmental factors that can affect the values. But in calculating the relative difference, those factors are largely eliminated as they apply to both cases. The relative GAIN is the important number.

One final note: it doesn't matter how strong or weak the returning signal is, it would not be possible to calculate the Moon's distance to centimeter accuracy were it not for the retroreflectors. I have posed this as a problem to a pathological doubter. Thus I shall not yet explain why. For those who want a hint, examine the problems that bedevil the Hams who engage in Moonbounce.

- NASA Laser Ranging Retroreflector Experiment
- McDonald Observatory Laser Ranging Station
- Interesting paper discussing alternate analysis of LRRR data, has much good information (PDF)